The monitoring of radioactive fibrinogen in order to detect the formation of blood clots (thrombii) in hospital patients has now become commonplace and is just one example of a practical use for a gamma counter. Briefly stated, the procedure employs fibrinogen, a component of the blood which aids in blood clotting. The fibrinogen s labeled with a radioactive element, typically Iodine-125, and then a scintillation counter is employed to detect the gamma rays which are emitted by the radioactive element as it decays. As is well known, a typical scintillation counter employs a photomultiplier tube and some form of phosphorous-type surface to receive the emitted gamma radiation. Quite frequently the phosphorous type surface is a sodium iodide crystal. The photons emitted by the sodium iodide crystal are detected and amplified by the photomultiplier tube, wherein the height of the pulse outputs of the photomultiplier tube are proportional to the number of photons emitted by the sodium iodide crystal.
The fibrinogen monitor, just like all gamma counters, must be calibrated before each use so that the data obtained may be meaningfully interpreted. Generally, the calibration involves the setting of an upper and lower counting-level limit and then the use of a monitoring detector and a sample of the radioactive label used as a reference source. The characteristic spectrum of the decay of the radioisotope being employed is already known and the calibration procedure involves positioning the upper and lower counting levels at certain locations in this spectrum, so that the peak of the spectrum occurs substantially at the midpoint between the upper and lower counting levels. Generally, the fibrinogen monitor is then operated and a histogram of the response is then plotted. This histogram is a spectrum of the pulse heights plotted in the pulse-height domain. The histogram is then reviewed to determine if the counting levels are positioned substantially around the midpoint of the spectrum. If it appears from the calibration histogram that the levels are not positioned around the peak of the spectrum, then the spectrum can be shifted by adjusting the high voltage on the anode of the photomultiplier tube. As is well known, the gain of a photomultiplier tube is based on the logarithm of its anode voltage. Continuing with the conventional calibration then, the anode voltage of the photomultiplier tube is adjusted and another histogram of the output is plotted. A review of this second histogram will again indicate whether the peak of the spectrum lies on the midpoint of the counting range as determined by the upper and lower levels. Quite frequently the anode voltage will have to be adjusted three or four times and the sample remeasured until the resultant histogram indicates the correct location of the spectrum peak vis-a-vis the upper and lower counting levels. It is, of course, appreciated that the plotting of the histogram is a relatively time-consuming process and quite frequently requires at least one-half hour to properly plot each histogram.